Multijunction (MJ) solar cells are the state of the art, high efficiency solar cell technology, having theoretical maximum efficiencies of ˜63% for a triple junction and ˜86% for an infinite series of junctions. See A. De Vos, “Detailed Balance Limit of the Efficiency of Tandem Solar-Cells,” J. Phys. D: Appl. Phys., vol. 13, pp. 839-846 (1980). MJ solar cells currently hold the highest conversion efficiency recorded, having demonstrated conversion efficiencies above 44.7% under concentrated sunlight. See F. Dimroth et al., “Wafer bonded four-junction GaInP/GaAs//GaInAsP/GaInAs concetrator solar cells with 44.7% efficiency,” Progress in Photovoltaics: Research and Applications, vol. 22, pp 277-282 (2014).
A conventional, two-terminal monolithic MJ solar cell consists of semiconductor layers grown sequentially on top of each other to form two or more series connected subcells. The subcells absorb incident sunlight and convert the light to electricity, and maximum efficiency is achieved when the band-gaps of the subcell materials split the incident solar spectrum optimally among the subcells.
A major technical challenge for such MJ solar cells lies in growing the multi-layered stack with high crystalline quality, which is extremely important for the efficient conversion of sunlight to electrical power. High crystalline quality is most easily achieved when the materials are grown lattice-matched to the growth substrate. Lattice mismatched growth, whilst enabling greater flexibility in available bandgaps, generally leads to high densities of dislocations and other defects that short the device and/or increase the parasitic loss for the photogenerated carriers.
Tunnel junctions, also known as Esaki diodes, are an important component of MJ solar cells, connecting the subcells of a monolithic MJ stack in electrical series. For optimal performance it is important that the tunnel junction (TJ) has a high enough peak tunnel current density to not impede the flow of photocurrent between the subcells, which can reach tens of A/cm2 in sun-concentrator applications. F. Dimroth, “High-efficiency solar cells from III-V compound semiconductors,” Phys. Status Solidi C, vol. 3, pp. 373-379 (2006). Also, the differential resistance of the TJ should be as low as possible to minimize any voltage drop across the TJ. A final consideration for solar cell applications is that the TJ should be as transparent as possible to light with energy below the band gap of the cell directly above the TJ, both to minimize the filtering of the light to the cell beneath and also to minimize the possibility of photocurrent being produced by the TJ.
In recent work, a basic p″/n++ TJ was produced using InAlGaAs material with a bandgap of 1.18 eV for an InP based MJ cell. See M. P. Lumb, M. K. Yakes, M. Gonzalez, I. Vurgaftman, C. G. Bailey, R. Hoheisel, and R. J. Walters, “Double quantum-well tunnel junctions with high peak tunnel currents and low absorption for InP multi-junction solar cells,” Appl. Phys. Lett., vol. 100, p. 213907 (2012). The bulk TJ was compared to a comparative lattice-matched quantum well tunnel junction (LM-QWTJ) structure, containing a pair of lattice-matched In0.53Ga0.47As quantum wells providing much more efficient interband tunneling. The relative performance of the LM-QWTJ and the bulk TJ were measured, with the QWTJ demonstrating a greater than 45-fold increase in the peak tunnel current and a greater than 15-fold reduction in the differential resistance, with only a small impact on the transparency. This result demonstrates that LM-QWTJs are able to reduce the voltage losses associated with the TJ resistance, without sacrificing the transparency of the TJ which would be compromised using bulk materials of the same bandgap as the QWs. This result is particularly important in high concentration applications where having minimal series resistance plays a crucial role in the power conversion efficiency.
The LM-QWTJ approach has intrinsic design flexibility by tailoring the dimensions and materials employed in the QWs and barriers. However, the tuning range is limited in scope due to the constraints imposed by the condition of lattice matching on the availability of materials with suitable bandgaps. The tunability of the QWTJ can be improved by using lattice mismatched layers, making a much wider choice of bandgaps available on any given substrate.
Incorporating strain into the epitaxial growth can introduce dislocations if the critical thickness for a given epitaxial layer is exceeded, therefore any lattice mismatched layer introduced in to the crystal needs to be within its critical thickness to maintain high material quality. However, the cumulative stress on the crystal from multiple strained layers grown within their critical thickness can also lead to dislocation generation. A proven method to incorporate strained layers into a crystal without a cumulative build-up of stress and hence avoid relaxation is to use strain-balancing, where the compressive strain introduced by a layer with lattice constant larger than the host substrate is balanced by tensile strain from a layer with a lattice constant smaller than the host. See N. J. Ekins-Daukes, K. Kawaguchi, and J. Zhang, “Strain-Balanced Criteria for Multiple Quantum Well Structures and Its Signature in X-ray Rocking Curves,” Cryst. Growth Des., vol. 2, pp. 287-292 (2002).
For example, strain-balancing has been used in solar cells incorporating multiple quantum well (MQW) structures into the intrinsic region of a p-i-n or n-i-p diode, where a compressive strain caused by use of a lattice-mismatched material in a quantum well situated between n- and p-type layers is balanced by a tensile strain in the barrier layers of the structure. See N. J. Ekins-Daukes, K. W. J. Barnham, J. P. Connolly, J. S. Roberts, J. C. Clark, G. Hill, and M. Mazzer, “Strain-balanced GaAsP/InGaAs quantum well solar cells,” Appl. Phys. Lett. Vol. 75, No. 26, pp. 4195-4197 (1999); and U.S. Patent Application Publication No. 2003/0089392 “Photovoltaic Device.” Strain balancing is used in such structures to enable many periods of the well/barrier system to be grown without degrading the crystal quality of subsequent layers. In these devices the QWs are used as absorbers, the bandgap of which can be tuned using the compositions and thicknesses of the QW/barrier system. The MQW structure is grown unintentionally doped, and carrier transport across the region is mediated by drift transport from the built-in electric field of the diode.